The invention herein described relates to a drill bit primarily for use in subterranean excavation.
In the following specification the term ‘conical sectional surface’ is deemed to mean a frustum of a generalised cone, the profile of the surface of which intermediate the base of the cone and its vertex may be straight, but may also be a generalised curve and may be continuous or discontinuous.
Conventional drill bits used in subterranean excavation are generally elongate structures with a generally circular cross-section comprising three main parts: First, there is a cutting face which contacts the material to be excavated. This usually comprises a plurality of cutting elements, the movement of which against the material to be cut causes matter to be cut or gouged away. Secondly, there are connecting means, usually located at an opposite end of the bit to the cutting face, for connecting the bit to a source of movement usually a rotary drill string. Thirdly, a so-called gauge region, intermediate the cutting face and connection means, the purpose of which is to contact sides of the hole being drilled in order to stabilise the movement of the bit. The gauge region may be generally free from cutting elements and has a diameter which is of similar size to that of the bore of the hole being drilled. The gauge region may also be provided with channels in its surface to allow cut material and drilling fluid to move away from the cutting face. This may occur as a result of drilling fluid being supplied to the cutting face by separate means, the drilling fluid displacing drilling fluid already present at the cutting face and cut material, causing it to flow through the gauge region channels away from the cutting face. The gauge region may be of generally uniform diameter, particularly if the drill bit is to be used in drilling straight holes. Gauge regions which incorporate a linear taper, i.e. where the diameter of the gauge region is reduced proportional to distance from the cutting face, resulting in a generally frusto-conical gauge region, have also been used.
It is well known to steer a drill bit so that it traces a curved path in a desired direction. In this situation part of the surface of the gauge region may be forced against the wall of the drill hole. This is a major problem, as it not only causes the drill bit to become unstable, but it also causes energy to be wasted in unnecessarily eroding the drill hole wall and/or the said surface of the gauge region. As the surface of the gauge region is also generally free of cutting elements, (but may have a hardened low-wear coating or covering) it means that its impacting with the drill hole wall will cause significant wear.
One method envisaged of overcoming this problem is the use of a drill bit with a curved profile gauge region. However, a drill bit of this type is less effective than a drill bit with a constant gauge cross section when utilised within a straight hole or a straight portion of a hole. This is due to the fact that curved profile of the gauge region will result in a portion of the gauge region not contacting the hole wall and therefore preventing it from stabilising the bit in the normal way.
Thus, a drill bit with a curved profile gauge region and a drill bit with a constant cross section gauge region are suitable for drilling either bent holes or straight holes respectively, but less effective in straight holes or bent holes respectively.
The proposed invention seeks to ameliorate the disadvantages hereinbefore described.